The invention relates to compensating bridge circuits, particularly to bridge circuits used for measuring mechanical quantities.
When mechanical quantities are measured by electrical means, the measured quantity is converted by a transducer to a corresponding electrical quantity. Transducers are functionally usually based on the change in the value of a resistance, an inductance, or a capacitance, resulting from a displacement caused by the measured quantity. Since such displacements are mostly very small, the resultant change in magnitude of the electrical output quantity is generally also small. This applies for instance to the measurement of mechanical quantities by strain gauges in which the changes in resistance that require detection and measurement are in the order of 10.sup..sup.-4 to about 10.sup..sup.-8. Apart from a few special cases, the required measurement is mostly performed by causing the change in the electrical quantity to affect the balance of a Wheatstone bridge and by either indicating the output voltage of the unbalanced bridge by a pointer on a scale (deflection method) or rebalancing the bridge by adjustment of a resistor (null method). Whereas the deflection method is preferred for solving the majority of measuring problems since the apparatus requirement is low, the null method is better for effecting very precise measurements because it eliminates the effect of fluctuations in the supply voltage and generally contains fewer parameters which affect the degree of precision.
Many different kinds of compensating circuits for measuring physical quantities are known. One such circuit is shown in FIG. 1. A voltage V.sub.b obtained from an input bridge A comprising strain gauges S is compensated by the output voltage V.sub.bb of a second balancing bridge B containing a measuring potentiometer Mp. In this compensating circuit it is a drawback that even the slightest drift in the value of the resistors R in the balancing bridge may falsely indicate that the input bridge is out of balance, thus adversely affecting the precision of the measurement in a manner that is particularly objectionable in cases in which the magnitude of a measured quantity is to be observed for a longer period in relation to a previously fixed reference value. Another drawback arises when the measured quantity is required not in terms of the degree of imbalance of the bridge but as an absolute value. In such a case, the measuring potentiometer would have to be provided with a scale of its own depending upon the type of the input transducers and the measuring range. Naturally this would cause major complications.
Another known circuit arrangement is that shown in FIG. 2. Here the balancing bridge is fed with a voltage reduced by a potential divider R.sub.d1, R.sub.d2, R.sub.d3. This leads to better null stability and at the same time the scale division of the measuring potentiometer Mp can be calibrated by varying a resistor R.sub.d1 in the potential divider. The somewhat large number of precision components in this arrangement is an undesirable feature. This applies more specifically when balancing is to be effected digitally with the aid of a stepped resistor network in which the resistors are attributed values according to a predetermined scheme. It will be appreciated that a linear potentiometer circuit contains twice as many components as a variable liner resistor.
Yet another compensating circuit is diagrammatically shown in FIG. 3. Here the potential divider has been replaced by a variable resistor R.sub.k. The circuit relies on a high ohmic differential amplifier with the gain g amplifies the input voltage V.sub.b. When the voltage g .sup.. V.sub.b will be compensated, the voltage across the null type indicator will be zero and the current i.sub.m flowing through the measuring resistor R.sub.m will be ##EQU1##
On the other hand, the relationship ##EQU2## holds for the compensating branch to which the supply voltage U.sub.S is applied.
It follows that ##EQU3##
The reading of the measured quantity therefore depends linearly upon the conductance G.sub.k of the balancing resistor R.sub.k. The measuring range can be easily varied by R.sub.m. On the other hand, an unfavorable factor is the incorporation of the amplifier in the measuring circuit. Its properties substantially affect the precision that can be achieved. For instance, the input resistance must be sufficiently high to keep the load on the transducer bridge A negligibly low. Moreover, the output resistance and the amplification must not be liable to drift. The only remedy is a high degree of feedback in the amplifier. Under the most unfavorable circumstances, the output voltage will then be determined only by the quality of the feedback network. This therefore also determines the precision that can be achieved, besides the resistors R.sub.m and R.sub.k. Another drawback, particularly when the signals are very weak is the background noise of the measuring amplifier which is superimposed on the measuring signal.